1. Field of the Invention
The present invention relates generally to a secondary battery protection circuit, and more particularly, to a secondary battery protection circuit that permits charging of a secondary battery even when the secondary battery has fully discharged.
2. Description of the Related Art
Compared to the conventional nickel-cadmium storage battery or nickel-hydrogen storage battery, the light, compact lithium-ion battery that is one type of secondary battery delivers approximately three times as much operating voltage. The power, lightness and compactness of the lithium-ion battery has led to its widespread use in video cameras, portable telephones, PHS, lap-top personal computers and a wide array of other portable electronic equipments.
In order to ensure the safety and enhance the performance of these light but powerful batteries, protection circuits have been used.
FIG. 3 shows a circuit for driving a charge control circuit in a conventional lithium battery protection circuit. Between a terminal 1 and a terminal 2 a lithium-ion battery 1 to be charged is connected. Between a terminal 3 and a terminal 5 a battery charger 2 for charging a lithium-ion battery 1 is connected and charges the lithium-ion battery 1. The actual charging of the lithium-ion battery 1 is carried out under the control of the charge control circuit 9.
The circuit that drives the charge control circuit 9 includes a transistor 3, a transistor 4, a resistor 5 and a controller 6. The transistor 3 is provided for charge control drive, and is connected in parallel to a power line 7 that connects the lithium-ion battery 1 and the battery charger 2. The emitter of the transistor 3 is connected to the power line 7, and the base of the transistor 3 is connected to the power line 7 via the resistor 5. The collector of the transistor 4 is connected to a node which connects the base of transistor 3 and the resistor 5. The emitter of transistor 4 is grounded. To the base of transistor 4 the controller 6 is connected, and to the controller 6 the charging terminal 5 is connected.
A description will now be given of the charging operation. The negative potential of the battery charger 2 is applied to the base of transistor 4 as a positive potential via the controller 6. As a result, when the transistor 4 is turned ON a current I.sub.0 flows. A voltage drop is generated across both ends of the resistor 5 by the current I.sub.0 and the base of transistor 3 is forward biased, turning ON the transistor 3. When the transistor 3 is turned ON a positive gate signal is output from the collector of the transistor 3. This gate signal is applied to the charge control circuit 9 to drive the charge control circuit 9 and begin charging the lithium-ion battery 1.
However, the conventional circuit as depicted in FIG. 3 requires the transistor 3 to be turned ON in order to drive the charge control circuit 9. Further, in order to drive this transistor 3 it is necessary to turn ON the transistor 4. Moreover, in order to turn ON the transistor 4 the collector-emitter voltage required to turn ON the transistor 4 must be applied between the collector and the emitter.
In order to achieve the activations described above, the lithium-ion battery 1 must retain the collector-emitter voltage necessary to turn ON the transistor 4. However, if the lithium-ion battery 1 is almost completely discharged then the residual voltage of the battery is not enough to turn ON the transistor 4. In that case the transistor 4 cannot be turned ON and, accordingly, the transistor 3 also cannot be turned ON. As a result, no drive signal can be output from the collector of the transistor 3 to the charge control circuit 9.
It is possible to charge the lithium-ion battery 1 even when the residual voltage of the battery is 0 V if a circuit configuration like that shown in FIG. 4 is used. In that case, connecting a battery charger 2 between a terminal 3 and a terminal 5 causes a current I.sub.1 to flow, a transistor 3 is turned ON by a voltage drop across the resistor 5, a positive drive signal is obtained from the collector of the transistor 3 and a drive signal can be applied to the charge control circuit 9.
However, the circuit described above consumes power. When the battery charger 2 is connected between the terminals 3 and 5, the current I.sub.1, flows through resistors 5 and 8 such that a power described by formula (1) is consumed, where R.sub.5 is the resistance of resistor 5 and R.sub.8 is the resistance of resistor 8. EQU I.sub.1.sup.2 (R.sub.5 +R.sub.8) (1)
It should be noted that the circuit shown in FIG. 4 can still operate even if the resistor 5 is removed, although the same problem of power consumption remains due to the operation of resistor 8.
It is possible to externally connect a logic circuit on the protection circuit so as to obtain a positive drive signal from the collector of the transistor 3 even when the residual voltage of the lithium-ion battery 1 to be charged is 0 V. However, in that case the scale of the circuit increases substantially.